Mature B cells comprise 10 to 15 percent of human peripheral blood lymphocytes, 50 percent of splenic lymphocytes, and approximately 10 percent of bone marrow lymphocytes. Mature B cells are derived from bone marrow precursor cells that arise continuously throughout life. B cells express on their surface intramembrane immunoglobulin (Ig) molecules that function as B cell antigen receptors in a complex of Ig-associated a and {character pullout} signaling molecules with intracellular signaling events. B cells also express surface receptors for the Fc region of IgG molecules as well as receptors for activated complement components. The primary function of B cells is to produce antibodies.
B cells undergo several selection steps during differentiation from immature cells to mature B cells or antibody-secreting cells. Only B cells which have undergone the selection process join the mature B cell pool and differentiate into antibody secreting cells upon stimulation. These selections allow for maintenance of a stable B cell pool without the production of autoreactive B cells which react to create an autoimmune response and efficient antibody production with adequate specificity and affinity. von Boehmer, H. Cell 1994 76:210; Liu et al. Immunol. Today 1992 13:17; Liu et al. Nature (Lond.) 1989 342:929. The selection steps involve apoptosis, survival or proliferation of B cells. A study in double transgenic mice revealed that apoptosis is important in the negative selection that destroys B cells bearing aberrant specificity against self-antigens. Hartley et al. Cell 1993 72:325. The autoantigen itself or the autoantigen in combination with other signals triggered apoptosis in these hazardous cells. In contrast, positive selection requires a condition where only cells receiving a signal are able to survive and grow. Otherwise, cells undergo apoptosis. In the germinal center, only cells bearing a surface immunoglobulin with sufficient affinity against an antigen are allowed to survive and grow. Liu et al. Immunol. Today 1992 13:17. In this situation, stimulatory signals such as the antigen itself or the CD40 ligand are shown to protect cells from apoptosis and induce survival or proliferation. Liu et al. Immunol. Today 1992 13:17; Liu et al. Nature (Lond.) 1989 342:929. Thus, apoptosis plays an essential role in the selection process. A signal that is capable of inducing or blocking apoptosis has been implicated as a pivotal system for selecting B cells.
Lymphocytes such as B cells are one of the most sensitive cells to immediate radiation induced damage. Anderson, R. E., and Warner, N. L. Adv. Immunol. 1976 24:215. Quiescent lymphocytes have been found to be more sensitive than actively cycling cells. The damaged cells are believed to undergo apoptosis. This process, which does not involve cell division, is sometimes referred to as interphase death. It has been hypothesized that lymphocyte signaling systems which are important in facilitating the B cell selection process are also related to cell death by irradiation.
Miyake et al. J. Exp. Med. 1994 180:1217-1224 disclose a monoclonal antibody, RP/14, that protects murine B cells from apoptosis induced by irradiation or dexamethasone. A molecule recognized by this antibody was found to be expressed on the murine B cells. This murine B cell surface molecule, referred to as RP105, has been further characterized. Miyake et al. J. Immunol. 1995 154:3333-3340. RP105 is a murine B cell antigen. It is monomeric, with a size of approximately 105 kDa. It is expressed on mature B cells, but not on either immature or pre-B cells. This antigen is believed to transmit a signal into murine B cells that results in protection from radiation or dexamethasone induced apoptosis.
The N-terminal amino acid sequence of the murine RP105 molecule has been determined. A cDNA clone was also isolated with a probe corresponding to the obtained amino acid sequence. DNA sequencing revealed that an encoded murine polypeptide is a type 1 transmembrane protein consisting of 641 amino acids in a mature form. Northern hybridization with the clone detected a transcript of approximately 3 kb. This transcript was found in mouse spleen, but not thymus, kidney, muscle, heart, brain or liver. Transfection of the clone into a pro-B cell line resulted in the expression of RP105.
A computer search showed similarity of murine RP105 to a number of molecules including decorin and biglycan, which are human proteoglycans in extracellular tissue; the Drosophila toll, tartan, connectin, chaoptin and slit proteins (which are responsible for dorsal/ventral polarity, epidermal/subepidermal structure, target recognition of a subset of motor neurons, photoreceptor morphogenesis and pathway finding by commissural axons during embryogenesis); and the a-subunit of platelet glycoprotein Ib (involved in platelet adhesion to vascular endothelial cells). Miyake et al. J. Immunol. 1995 154:3333-3340. The property shared by murine RP105 and these molecules is tandem repeats of a leucine-rich motif (LRM) These repeated motifs are observed in members of the leucine-rich repeat protein family and have been implicated in protein-protein interactions such as cell adhesion or receptor-ligand binding. The murine RP105 molecule has 22 tandem repeats of a leucine-rich motif, as well as amino and carboxyl flanking regions that are characteristically conserved among members of this family. Thus, murine RP105 is believed to be a member of the leucine-rich repeat family and the first one that is expressed specifically on mature B cells.
A human gene encoding a novel human B cell surface molecule has now been found.